The present invention relates to a testing device for performing four-point fatigue strength tests of a sample, such as a printed circuit board, by applying alternating bending stresses to the sample by way of rotary joints held by holders mounted in a force introducing machine, such as a universal material testing machine. A testing device of this type is disclosed, for example, in the in-house publication of Roell+Korthaus, Amsler-Prufmaschinen AG (Schaffhausen, Switzerland), entitled "Prufsysteme" [Testing Systems], Specification 205, pages 8-9, under the heading "4-Punkt-Biegevorrichtung" [Four-Point Bending Device]. Testing devices of this type are employed for mechanical materials testing, particularly to test the service life and service reliability of semi-finished products and components, particularly printed circuit boards.
The prior art four-point testing device for alternating bending made by Roell+Korthaus is composed of four clamps for holding the sample. The clamps are held in a pivotal manner by spring elements. However, this way of clamping in the sample generates structurally caused, undesirable tensile and compressive forces in the sample in addition to the bending stresses. As a result, the stroke of this testing device is limited to .+-.0.6 mm according to the manufacturer's information.
An article by K. G. Kiessling, entitled "Untersuchungen zur Zuverlassigkeit von Lotverbindungen in der SMD-Technik" [Tests Regarding the Reliability of Solder Connections in SMD (Surface Mounted Device) Technology], in SMT/ASIC/Hybrid 1990 Congress, May 15-17, 1990, Nurnberg (published by Huthig Buch Verlag, Heidelberg, 1990), pages 423-431, further describes a testing device for performing three-point fatigue strength tests under alternating bending stresses. Testing devices of this type have the drawback that they stress the samples to be examined with a bending moment that changes linearly over the length of the sample.